With the ever-increasing demand for portable communication devices, reliability and efficiency of both user devices and devices in the supporting network has become of increasing importance. There are a number of different considerations, related to both individual elements as well as system elements, which affect these characteristics. For example, it is desirable to increase both bandwidth efficiency and power efficiency of a power amplifier in various communication devices. While bandwidth efficiency (the rate that data can be transmitted over a given bandwidth) is typically achieved using linear modulation, amplifier efficiency is a significant concern for achieving longer battery life and lower energy costs in transmitters as it usually dominates the power consumption in the system.
A transmitter of the portable communication device or of a communication device in the infrastructure (such as a base station) generally uses a radio frequency power amplifier (RFPA) as the final amplifying stage of a transmitter. The RFPA typically has a fixed power supply voltage. With a fixed supply voltage, however, the efficiency of the RFPA decreases as the output signal magnitude drops, leading to ineffectiveness and excessive peak power capability. To improve efficiency, it is desirable for the RFPA to continually operate near saturation, where the amplifier is close to or slightly gain compressed (about 0.5 dB below gain compression to 0.5 dB in gain compression). This can be achieved by modulating the power supply of the RFPA using the known technique of envelope tracking (that is, the supply voltage of the RFPA tracks the output signal of the RFPA), which adjusts the power supply of the RFPA such that the power supply voltage of the RFPA follows the output signal thereby allowing the RFPA to continually operate near saturation.
To prevent starvation of the RFPA, some margin is designed into the implementation. However, adding margin to the supply voltage of the RFPA is less than ideal because it reduces average efficiency. At the same time, not having the margin will often cause distortion in the amplified signal generated by the RFPA because of unexpected gain compression. In addition, for an adaptive design, the RFPA is often designed to handle several different signal modulation schemes. Therefore, it is also required that today's efficiency improvement methods be able operate for any given modulation. Correspondingly, amplitude accuracy of a reference envelope of the modulation and time alignment of the envelope signal provided to the RFPA are very important.
In a practical application, an amplitude and/or phase of an I/Q path is modified to improve overall transmitter performance, causing the overall amplitude of the envelope signal (I^2+Q^2) to be distorted from a received input signal that is to be transmitted. The envelope signal can be single ended or differential in nature. Such modifications may include DC offset calibration, phase calibration and IQ balance calibration. In addition to these changes, in a dynamic system that is capable of operation over a wide output power range, the amplitude of the I/Q path out of a DSP often is adjusted in order to provide the full output dynamic range. In a supply modulated system, the envelope of the signal to be transmitted is the reference that ultimately sets the minimum and maximum voltage levels of the modulation to the RFPA.
In addition to amplitude calibration, time alignment of the envelope signal to the transmitted RF signal is critical in supply modulated systems for high efficiency. In a practical application, the delay in the I/Q path is added by filters required for signal conditioning, such as low pass and switch capacitor filtering. In addition, the switch capacitor filters often are programmable to allow adjustments for a particular modulation or bandwidth. Therefore, in practice, the programmable delay must be matched in both the envelope and I/Q paths. However, adding an additional fixed hardware delay many not always be possible due to space limitations, additional cost for the hardware delay elements and once designed, it is often very difficult to add additional delay in either path if required.
Therefore, a need exists for an improved reference accuracy scheme comprising of both amplitude and time alignment of the envelope and RF signals. In addition, the improved reference accuracy scheme should provide a simple means for mass production and that is programmable to allow time alignment independent of the amount of fixed delay in the I/Q path in a supply modulated system.
One of ordinary skill in the art will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.